In conventional phase-sensitive optical time domain reflectometry (φ-OTDR), the length of sensing fiber mainly
determines the repetition rate of probe light pulses, which limits the extent of detectable frequency range. Moreover,
averaging method, which is adopted to enhance the location signal-to-noise-ratio (SNR), further decreases the maximum
detectable frequency. This paper demonstrates a distributed vibration sensor with satisfied location SNR and extended
frequency response range by using a probe pulse pair with a frequency difference. Experimental results show that this
method is able to break the trade-off between the given sensing fiber length and the traditional maximum detectable
frequency response of φ-OTDR system.
The effect of fiber laser linewidth on the optical time-domain reflectometer based vibration distributed sensing regime is studied experimentally in this paper. Lasers with multi-longitudinal modes, 5 kHz and ~800 Hz linewidth single longitudinal mode are interrogated as light source successively. Experiments show that narrower linewidth laser results in higher repeatable and stable scattering spectrum. When the 800 Hz linewidth laser is modulated into 50 ns width pulses, the signal to noise ratio is ~ 6 dB and the spatial resolution is improved to 3 m, breaking the traditional theory limit.
An optical fiber distributed sensing system merged Mach-Zehnder interferometer (MZI) and phase-sensitive optical time domain reflectometer (φ-OTDR) for vibration measurement with wide frequency response and spatial resolution is proposed and demonstrated. Two acoustic optical modulators (AOM) are adopted to generate narrow pulses and wide pulses respectively on both ends of sensing fiber with a time difference. Narrow pulses are used to generate Rayleigh backscattering light, which locates the vibration point, while wide pulses interfere with reference light as a MZI to obtain frequency response. To simulate the high frequency responses of crack in civil structures, the sudden break of pencils adjacent to fiber loop has been measured. The experimental results show 5 m spatial resolution and up to 6.3 MHz frequency response with 50 ns pulse width are achieved in 1150 m sensing distance.